Field of the Invention
The present invention relates to a rack and an optical apparatus having the same.
Description of the Related Art
Downsizing of optical apparatuses, such as a digital still camera and a digital video camera, requires downsizing of a lens barrel mounted onto them. Power saving of the lens barrel is also required in order to downsize a battery and extend the maximum duration of shooting. Japanese Patent Laid-open No. 2005-315935 discloses a movement mechanism including a motor and a rack to move a lens holding frame in an optical axis direction and a torsion coil spring to force the rack against a lead screw of the motor in a lens barrel.
In a rack configured to hold a lead screw using a main tooth and a pinching tooth, an impact due to a drop, for example, causes a tooth jump of the main tooth and the pinching tooth off a thread of the lead screw, resulting in disturbance and blur in a captured image. When a strong force is applied between the rack and the lead screw to prevent a tooth jump as disclosed in Japanese Patent Laid-open No. 2005-315935, an increase in fractional force between them leads to an increase in a rotational load on the motor, thereby hindering power saving of the motor, or increasing the size of the motor and hence the size of the lens barrel. In contrast, in a rack configured to engage only a forced main tooth with a lead screw in a normal state, an opposite tooth disposed opposite the main tooth with respect to the axis of the lead screw becomes engaged with the thread of the lead screw when an impact is applied, thereby preventing the rack from a shift in position in the axis direction of the lead screw. With an extremely strong impact, however, a strong shearing force is applied on the tip of the main tooth to potentially chip off the main tooth, which can causes operational failure.
Along with recent downsizing demands of the digital camera and digital video camera, downsizing of the rack also becomes necessary.